I am trying to set a distributor into a 350 GMC p/u the problem:?

That's a great answer krooser2

Don't overlook the 18436572 counter clockwise plug wire placement. #1 belongs at the rear of the access cover on the cap

First make sure the #1 cylinder is at Top Dead Center (TDC). Pull the #1 spark plug (first on the drivers side) , ground the plug wire and place your index finger over the spark plug hole. Have someone 'bump' the starter until you feel the compression starting to build on your finger... then look at the ignition timing pointer on the timing cover (front of the engine)...when the pointer and the TDC mark are aligned then you are at TDC.

Then find the # plug wire on the distributor cap and install the distributor so the distributor rotor points to the #1 plug wire. You may have to put slight pressure on the distributor while someone 'bumps' the starter which will allow the distributor to drop into place. Get the #1 plug wire to align with the distributor rotor and reinstall the dist. cap. Re-install the #1 spark plug and you should be able to fire the engine. You will have to make SMALL adjustments to the distributor by turning it slightly until the engine runs smooth. Time it with a timimg light and you should be all set.

the distributor also turns the oil pump make sure the slot it the oil pump shaft is in line with the notch on the bottom of the distributor u may need to turn the pump shaft with a long screw reg screwdriver.its easy to be of a tooth.